出现如此异常，老手帮忙看下

严重: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at org.apache.struts.taglib.TagUtils.getActionMappingName(TagUtils.java:583)
at org.apache.struts.taglib.html.FormTag.lookup(FormTag.java:834)
at org.apache.struts.taglib.html.FormTag.doStartTag(FormTag.java:467)
at org.apache.jsp.index_jsp._jspx_meth_html_005fform_005f0(index_jsp.java:150)
at org.apache.jsp.index_jsp._jspx_meth_html_005fhtml_005f0(index_jsp.java:125)
at org.apache.jsp.index_jsp._jspService(index_jsp.java:75)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:393)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:263)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:584)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)帮忙看下是何原因，谢谢